When you focused with the outer points, does it give you the same results with all the outer points, ie left and right sides?
I just got my lens a few days ago. I seemed to have a pretty good one. With +0 MA it is sharp and AF is consistent with all focus points on my 5diii, all outer and center.
Then I tried calibration with the dock. I observed it was a tad sharper with a +1 calibration at MDF and 2.2 ft. The 3rd and infinity point was perfect at +0. (Ma at 1,1,0,0)
Well this caused my lens / 5diii combo to go nuts with AF. Between MDF and around 2.2 feet, it mis-focused thinking it was 8ft away rather than two feet away. However, the mis--focus was consistent.
So by trial and error, I found best that I set the four MA points to 0,1,0,0. Now I get sharp and consistent focus through all distances and no issues thus far. I'm pretty happy with this lens. The +1 at MDF is slightly better, but is pretty anal. (It's in between 0 and 1) The +1 at 2.2 feet is noticeable at pixel peeping. When sigma upgrade the firmware, I'll try 1,1,0,0 again.